What is Galaxies: Definition and 417 Discussions

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant galaxy observed. It has a comoving distance of 32 billion light-years from Earth, and is seen as it existed just 400 million years after the Big Bang.
In 2021, data from NASA's New Horizons space probe was used to revise the previous estimate of 2 trillion galaxies down to roughly 200 billion galaxies (2×1011). This followed a 2016 estimate that there were two trillion (2×1012) or more galaxies in the observable universe, overall, as many as an estimated 1×1024 stars (more stars than all the grains of sand on planet Earth). Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 ly) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)
The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which it dominates along with Andromeda Galaxy. The group is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea.

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  1. mister i

    B Could Dark Energy be an (unknown) dark force acting at great distances?

    I am not a physicist but if someone can answer this question: It seems that there is a dark energy in the universe. Could it be associated with an (unknown) dark force acting at great distances?: I don't quite understand that when we talk about the approach of galaxies we talk in terms of force...
  2. Z

    B Attraction or repulsion for objects moving away from each other with acceleration?

    If galaxies fly apart from each other with ACCELERATION, why is the term "universal attraction" used and not "universal repulsion"?
  3. DavidCummings

    I Did James Webb cast serious doubt on the Big Bang?

    If you read articles like The James Webb Space Telescope prompts a rethink of how galaxies form you see that recent images of early galaxies have thrown some doubt on theories of how galaxies have formed and when they started forming. But if you read about the JWST in the popular media -- and...
  4. Cerenkov

    B What value of z (redshift) equals apparent superluminal recession?

    Hello. I have visited this site recently... https://lco.global/spacebook/light/redshift/ ...and was wondering at what value of z in the table below would galaxies appear to recede at superluminal velocities? z Time the light has been traveling Distance to the object now 0.0000715 1...
  5. S

    I Tidal effects of galaxies orbiting one another with dark energy?

    I recently found a question in a physics discussion site [1] about whether there was a "distance" between two galaxies where both the gravitational force and the influence of dark energy would be balanced. The answers and comments seem to indicate that there is indeed such a "radius" around a...
  6. H

    I Ancient galaxies: lack of mergers vs. subsequent spatial expansion

    Galaxies far away---that evolved soon after the Big Bang---are reported to be closer together than the galaxies we observe near us. This could be due to two distinct reasons. One is that relatively nearby galaxies, over time, tend to merge, and the ones we see from far away haven't had time to...
  7. bakerjay

    A Galaxies where V(obs) is less than V(bar)

    I've been looking through the SPARC dataset (in particular, this part: http://astroweb.case.edu/SPARC/MassModels_Lelli2016c.mrt), and looking at the observed velocities vs velocities expected from baryonic matter. While in some cases I see the sort of rotation curves I'd expect, with observed...
  8. gleem

    I Massive galaxies during the early Universe, new JWST data

    As of now, it appears the ΛCDM can accommodate this new data but new data is needed to be sure. https://www.quantamagazine.org/standard-model-of-cosmology-survives-jwsts-surprising-finds-20230120/
  9. A

    I Measuring the rotation curve of galaxies

    What should I learn to make astrophysical measurements from open data? Suppose I want to measure the rotation speed of galaxies to generate galactic rotation curves like these: https://en.wikipedia.org/wiki/Galaxy_rotation_curve What should I do and what should I learn? I think I should get...
  10. G

    B How did Earth get so far away from other galaxies?

    If the Big Bang happened 13.6 billion years ago how can the James Webb telescope now see galaxies 13.3 billion light years away? 13.3 billion years ago the universe was 0.3 billion years old. How large was the universe at that time? If physical mater traveled in all directions from the point...
  11. G

    B Can physical constants be different in other galaxies?

    I've learned that some cosmological observations do not match with theoretical predictions leading to the hypothesis of dark matter or dark energy. Do you know references which discuss if instead the explanation could be that physical constants slowly vary in space? This seems conceptually...
  12. cosmologyscience

    I Orbital Velocities and Mass Distribution in Galaxies

    Has anyone looked into the details of stellar orbital speeds and required (visible) mass distribution in the Milky Way? Doing some math here - if the local mass density is significantly higher in the inner 10-15% of the galaxy, and then lower and gradually thinning outwards in the disk, we will...
  13. T

    I Why Do Galaxies Spin? Exploring the Science Behind It

    I googled it, and almost every source I find says something like this: "These stars were then gravitationally attracted to each other to create gigantic clusters of stars enshrouded in clouds of gas. Eventually these groupings of stars come together through the attraction of gravity and...
  14. A

    B Rotation of Galaxies: Mass vs Dark Matter?

    Is it possible that galaxies are spinning faster than expected due to a particular configuration of the moving mass and not due to dark matter?
  15. K

    I MOND from galaxies that are fractal

    I found these papers, the first is that galaxies are fractal based on observation Fractal Analysis of the UltraVISTA Galaxy Survey Sharon Teles (1), Amanda R. Lopes (2), Marcelo B. Ribeiro (1,3) ((1) Valongo Observatory, Universidade Federal do Rio de Janeiro, Brazil, (2) Department of...
  16. T

    A Which galaxies have the best studied evidence for dark matter?

    Unsurprisingly the Milky Way seems to have been studied a lot. We have really good luminous mass profiles (e.g. McMillan 2011) and increasingly accurate circular velocity observations for stars at various radiuses (e.g. Eilers 2019) meaning we can confidently infer dark matter models. So the...
  17. PainterGuy

    B The configuration of galaxies in space

    Hi, In the picture below, M is Milky Way Galaxy in red, both A's are Andromeda Galaxy in positions P1 and P2 , and W is some random galaxy. Milky Way Galaxy lie in one plane just like all the planets in solar system. In what configuration Andromeda Galaxy lies? Does it lie almost at the same...
  18. C

    B No way to catch up with galaxies currently receding at >c?

    Consider a far-away galaxy that is considered to be currently receding from Earth at 2x the speed of light. (With this 2x c recession velocity, we are speaking of the Vnow, the imputed relative velocity of the galaxy compared to Earth now, not the velocity of the galaxy relative to Earth at the...
  19. elcaro

    B Can we measure acceleration of galaxies and stars?

    As for example we see a large void, the Great Repeller, which in fact is an underdense region, and with respect to this region, matter seems to be repelled by this region. The explenation for that is that matter outside that regions pulls on the matter inside it. But if that is really the...
  20. M

    I ##\Lambda##CDM: Evolution of the population of galaxies

    The number of visible galaxies is to vanish in the long-term future with the ##\Lambda##CDM model (book and papers of L. Krauss et al). I am interested in a quantitative study on how the population of galaxies decreases; I could not find one so I do it myself: The scale factor...
  21. M

    I The Acceleration and the density of galaxies

    In the Lambda-CDM model, the density of galaxies goes decreasing and should even vanish in the far future. I would be grateful if someone could point me to a paper where this is calculated.
  22. bbbl67

    I Andromeda vs. Milky Way galaxies

    Has their been any explanations found for why the Andromeda galaxy is so much bigger in diameter than the MW, yet at the same time it's lighter? Is the Andromeda a type of Low-Surface Brightness (LSB) galaxy? Also maybe somewhat related, why is the Andromeda's central black hole so much...
  23. seabass101

    I Hubble deep field & ancient galaxies

    Hubble deep field allowed us to study galaxy evolution from 500 million years onward. Based on my (limited) understanding, I would expect ancient galaxies to contain fewer heavy elements and to have a more "juvenile" appearance, as compared to modern galaxies. Have we actually observed these...
  24. M

    What kind of telescope can see planets in other galaxies?

    I was recently looking at a comic reprinting of Retro Sci Fi Tales # 9, and the synopsis on the site spoke about a story of the "Exposition Universelle", where at a fictitious worlds fair in Paris in 1878, they unveil a "grand inter-galactic telescope so powerful that it can view the surfaces of...
  25. M

    B Nervous about James Webb Space Telescope?

    The Hubble Telescope helped us to see how enormous the universe really is. We now know from data built up from that that the universe likely has 2 trillion galaxies in it. Now when James Webb gets out their and starts taking better pictures; I’m afraid the count of galaxies will jump to 10 or...
  26. Cerenkov

    B Trying to understand the differences between distant & local galaxies

    Hello. The recent discovery of the galaxy SPT0418-47 has piqued my interest. https://www.almaobservatory.org/en/images/reconstructed-view-of-spt0418-47/ https://en.wikipedia.org/wiki/SPT0418-47 https://www.eso.org/public/archives/releases/sciencepapers/eso2013/eso2013a.pdf It's my current...
  27. D

    I How fast do galaxies 1 billion light-years away move away from us?

    We know that galaxies that are 1 megaparsec away from us (3.2 million light-years) are moving away from us here at a speed that is approximately 70km/s. Given that that is the case would that also imply that galaxies that are 312 megaparsecs (1 billion light-years) away from us are moving away...
  28. gptodd

    I Apparent distance to far galaxies

    If we had a way to measure the apparent distance to 13billion light year galaxies (parallax or something) what would it be? The light left the source 13 billion years ago, so it would appear much closer. I can't find an answer and I'm hoping you know.
  29. akshitha135

    B Are there any blue shifted galaxies name rc2?

    where can I find a list of blue-shift galaxies, thanks.
  30. Buzz Bloom

    I Question regarding the future of galaxies within a group of clusters

    Given that the galaxies within a cluster of galaxies are generally gravitationally bound, and not affected by the expanding universe, would it not also be expected that after some large number of billions of years, all of the individual galaxies would merge together to become one single very...
  31. D

    What percent of the universe is outside of galaxies?

    Summary:: What percent of the universe is outside of galaxies? Hello, What percent of the universe is outside of galaxies? I need this information for a writing piece I am doing. I need specifically to be able to state that if someone was to randomly relocate to somewhere else in the...
  32. Raimund Kempe

    How can the rotation of galaxies be explained?

    How did you find PF?: Google search How can galaxies rotate coming from a singularity? Raimund
  33. S

    B Old galaxies in every direction

    Look, I had to digest a lot from answers to first question. Still, I'm missing some main idea. As far as we look in any direction we can see stuff out there from long ago. A grenade explodes in some void. The material travels outward as a sphere of leading faster bits. Slower material follows...
  34. E

    Comparing Spectra of Distant Galaxies

    So tell me if I'm wrong, the left spectrum (white) and the right one (red) show spectra of two different galaxies (with different distances), right ? Why is it possible to know the velocity of the red-shifted spectrum by comparing it to the left one ??
  35. M

    I Why does the Universe have so many galaxies?

    I know this question doesn’t really make sense but the absurd enormity of the universe always made my head hurt. A single galaxy alone has more then enough stars to satisfy the curiosity of any sentient being. Why 2 trillion? It’s just insane. Why did nature have to create so many? The spaces...
  36. M

    B Dark Matter Distrbution in Galaxies

    Do we understand Dark Matter enough to know how it is concentrated within a galaxy. For example, would it be evenly distributed like hydrogen would be or could it be concentrated like most matter is? For example could there potentially be Dark Matter stars and Dark Matter planets within our galaxy?
  37. Ranku

    I Dynamical friction in galaxies

    Analysis of dynamical friction in clusters of galaxies produces mass segregation between heavier matter at the center and light matter diffused in the outer regions (Ref). Can the flat rotation curve in an individual spiral galaxy, whereby heavier matter is concentrated at the center and...
  38. Powehi

    B Superclusters and their galaxies

    I’ve been looking pretty much everywhere on the internet but I can’t find any answer to this question. How many galaxies are in average in a supercluster ? Knowing that laniakea has 47000 galaxies is it considered as a big or a little supercluster?
  39. H

    I Is there a radial gradient of redshifts within individual galaxies?

    The summary says it all. Such small gradients, if they exist, would be visible in the Milky Way and local galaxies in our cluster. I'm not familiar enough with the raw data--and haven't tried to search the astronomical literature--to know whether any such small effect has been reported. (If...
  40. F

    I Formula for the large-scale bias of galaxies

    From this article : https://arxiv.org/pdf/1611.09787.pdf , I try to deduce the equation that my teacher told me which links 2 quantities : 1) the global number density of galaxies 2) the local number density of galaxies 3) the contrast of Dark matter density The relation that I would like to...
  41. C

    B Milky Way is "in the top percentile of all the galaxies that exist"

    Our Galaxy is far bigger, brighter, and more massive than most others: Knowable Magazine Check out the amazing video, which shows all the many--more than 50!--satellite galaxies of the Milky Way burst into existence as they are discovered, year after year.
  42. Zeynel

    Spiral Galaxies and Cyclones: Investigating Their Similarities

    There’s an obvious resemblance between a spiral galaxy and a cyclone. They have the same spiral shape. I was wondering if the equations describing both motion are the same. I googled vortex motion but equations and concepts looked very specialized and complex for me to understand easily. Thanks.
  43. Z

    I Why are pictures of galaxies so clear?

    When a photo of a nearly edge on galaxy, say Andromeda, is taken. Why are the stars on the far side of the galaxy so clear? The light from those stars took an additional 220,000 years to reach us. Those stars have moved quite a lot from their positions since the light from the foreground stars...
  44. nmsurobert

    I Gravitationally bound galaxies

    How do we know if galaxies are gravitationally bound? I'm guessing it obviously has something to do with the mass of each galaxy in the cluster, but is there an equation that is used to determine when they are bound to each other? Is there some kind of measurement made regarding the velocity...
  45. Buckethead

    B Orbits of galaxies in the Coma Cluster

    Because of Zwicky'f findings back in 1933 on the Coma Cluster rotational curves there is something I was wondering about with regard to this cluster. Are the galaxies orbiting in generally the same direction? (like a spiral galaxy) or in completely random directions like an Elliptical, or...
  46. S

    I Uncertainty Over Sextans Galaxies' Group Membership

    Hi all. Awesome site! Just wondering if anyone can answer my question: If the Sextans galaxies are inside the group's zero velocity surface, why is there uncertainty over whether they're part of the group?
  47. Ranku

    I Universe without dark matter

    How different would the universe look without dark matter? Would galaxies have formed, allowing for the existence of the solar system and life?
  48. C

    Observing galaxies: area of sky would I need to survey

    Homework Statement Given that there are 10-2 Ellipticals per Mpc3 and my garden telescope can reach to 14 mag. How large an area of sky would I need to survey to find 100 Elliptical galaxies ? (assume the typical absolute magnitude for an Elliptical galaxy is -21 mag).Homework Equations...
  49. hyksos

    I Imaging galaxies receding at a velocity that exceeds c?

    There are galaxies that are so far away that metric expansion causes them to have a co-moving recessional velocity that exceeds the speed of light. However, those galaxies are also so far away that the time it took the light to reach us was itself billions of years in the passage of its journey...